The present invention relates to a liquid crystal display apparatus capable of displaying an image in various kinds of environments in ranges from a light place to a dark place and realizing a high-quality transmissive display at a wide viewing angle.
Today, a transmissive liquid crystal apparatus with a wide viewing angle of such as an IPS (In-plane Switching) or an VA (Vertical Alignment) liquid crystal display is prevailing as a monitor. Such a liquid crystal apparatus with improved response performance is also used as a television. On the other hand, the liquid crystal display apparatus is also prevailing in the field of portable instruments such as a portable phone and a digital camera. Though the portable information instruments are mainly used by persons, today, those instruments are more and more requested to have the display portion with a variable viewing angle, in which it is requested to have a wide viewing angle because the display is often watched in the oblique direction.
The display apparatus provided in the portable information instrument is used in various kinds of environments in ranges from an outdoor place in a fine weather to a dark room. Hence, the display apparatus is requested to be transflective. The transflective liquid crystal display apparatus includes a reflective display portion and a transmissive display portion on one pixel. The reflective display portion reflects a ray of light incident from the circumference thereof through the use of a reflective layer, for displaying an image. The reflective display portion keeps the contrast ratio constant irrespective of the brightness therearound, so that the display portion may provide an excellent display in a relatively bright environment in ranges from an outdoor place to an indoor place in a fine weather. The transmissive display portion keeps the brightness constant irrespective of the ambient light through the use of a backlight, so that the display portion may provide a display of a high contrast ratio in a relatively gloomy environment in ranges from an indoor place to a dark room. Therefore, the transflective liquid crystal display apparatus provided with both of the reflective and the transmissive display portions may provide a display of a high contrast ratio in various kinds of environments in ranges from an outdoor place in a fine weather to a dark room.
Up to this time, by producing the IPS liquid crystal display apparatus has been known as a transmissive display with a wide viewing angle, and it's potential for a transflective display has been expected to obtain both the reflective display and the transmissive display with a wide viewing angle. For example, the JP-A-2002-333624 discloses the reflective IPS liquid crystal display apparatus.
The transflective IPS liquid crystal display apparatus includes a plurality of phase plates located wholly on the upper side and the lower side of the liquid crystal panel. However, the phase difference of the phase plate depends upon a viewing angle. Hence, even if the phase difference between a plurality of phase plates and a liquid crystal layer and an axial alignment thereof are optimized in the normal direction, the optimal condition for the dark image is being quickly shifted as the liquid crystal layer and the phase plates are being off the normal direction. The dependency of the phase difference upon a viewing angle may be reduced by adjusting the index of refraction in the thickness direction of the phase plate. However, the dependency cannot be completely eliminated. As a result, the transflective IPS liquid crystal display apparatus has an increased dark image transmittance in the viewing angle direction, in which the viewing angle of the transflective display portion is lower than that of the transmissive IPS liquid crystal display apparatus.
Further, the JP-A-2002-207227 discloses that the alignment of the liquid crystal layer in the reflective display portion is different from that in the transmissive display portion in the VA system. However, it does not disclose the application of those different alignments to the IPS system that provides a transmissive display with a wide viewing angle.
In the transmissive IPS liquid crystal display apparatus, the liquid crystal layer is homogeneously aligned, the transmissive axes of the upper and the lower polarization plates are located to be crossed with each other at right angles, and one of the transmissive axes is located in parallel to the liquid crystal alignment direction. The ray of light incident to the liquid crystal layer is linearly polarized light and its electric vector is located in parallel to the liquid crystal alignment direction. Hence, the liquid crystal layer does not bring about the phase difference. This makes it possible to realize a dark image of a low transmittance, bring about no extra phase different in the viewing angle direction because of no phase plate located between the liquid crystal layer and the polarizers, and thereby realize the dark image of a wide viewing angle. As described above, in essence, the transmissive IPS liquid crystal display apparatus does not need to locate any phase plate.
The transflective liquid crystal display apparatus includes the reflective display portion and the transmissive display portion on one pixel, in which the optical condition for the dark image is essentially different between the reflective display portion and the transmissive one. That is, in the reflective display portion, a ray of light is entered from the polarizer located on the top of the liquid crystal display apparatus, reflected on the reflective plate inside the liquid crystal panel, passed through the polarizer located on the top thereof again, and then reaches the user. In the transmissive display portion, a ray of light is entered from the polarizer located on the bottom of the liquid crystal display apparatus, passed through the polarizer located on the top thereof, and then reaches the user.
The foregoing difference of a light pass causes the reflective display portion to have a light phase difference bringing about the dark image being different by a quarter wave from the transmissive display portion. Hence, when the reflective display portion stays in the bright image, the transmissive display portion stays in the dark image, or vice versa. It means that the reflective display portion has different dependency on the applied voltage from the transmissive display portion. In order to keep the dependency on the applied voltage identical in both of the display portions, it is necessary to shift the phase difference between the reflective display portion and the transmissive display portion by a quarter wave by any means.
The conventional transflective IPS liquid crystal display apparatus includes the phase plates located wholly on the upper and the lower surfaces of the liquid crystal panel. The phase plate located on the top of the liquid crystal panel passes through a ray of light entered from the outside to the reflective display portion, the ray of light reflected on the reflective plate of the reflective display portion, and the ray of light having been passed through the transmissive display portion. As described above, the upper phase plate is effective in both of the reflective display portion and the transmissive display portion. On the other hand, since only the source light entered into the transmissive display portion is passed through the phase plate located on the lower side of the liquid crystal panel, the lower phase plate is effected only in the transmissive display portion.
By using the foregoing difference of the effect between the upper phase plate and the lower one against the reflective display portion and the transmissive one, the phase difference therebetween is shifted by a quarter wave. However, since the phase plate is located between the liquid crystal layer and the polarizer, the extra phase difference occurs in the viewing angle direction, which leads to lowering the viewing angle performance on the dark image.